Measures of Disease Frequency PDF

Measures of disease Frequency/Occurance Dr Charles Mangani Community and Environmental Health Department  Learning Outcomes Explain why we measure disease frequency Categorise the measures of disease frequency Calculate the measures of disease frequency...

Measures of disease Frequency/Occurance Dr Charles Mangani Community and Environmental Health Department  Learning Outcomes Explain why we measure disease frequency Categorise the measures of disease frequency Calculate the measures of disease frequency Interpret the measures of disease frequency Natural history of disease in an individual Gordis Progression of disease in a population Disease Frequency-Why Measure? To understand the distribution of disease Measures of population health ✓ Community health assessment ✓ Surveillance, monitoring Evaluation of effectiveness control measures/interventions Assessment of epidemiological research ✓ Population burden for a specific illness ✓ Descriptive and analytical studies  Measures of disease frequency Quantification of the existence or occurrence of disease or event Simplest ,counts ✓ Number of cases e.g. 30,000 cases of malaria in 2022 ✓ Number of people receiving ARVs in 2022: 600,000 Problem with counts??  Need for denominators Define the population ✓ Where are the cases coming from? ✓ What is the source population, or population at risk? Allows comparability across populations “Populations” include not only countries, but geographical regions, demographic groups, communities, extended families, etc  Rates : how fast the disease is occurring in a population Proportions: what fraction of the population is affected  Prevalence Proportion of the population affected/with condition at any given time P= (C/N) where – C: Number of cases of disease present: new + old – N: population size Numerator: number of cases of disease in a population – Regardless of onset; new or old – Can be at a single time point(point prevalence) or time interval (period prevalence) Denominator: size of population at risk – Same time point/interval ranges from 0.0 to 1.0 but can be expressed as percentage  Point Vs Period prevalence Interview Question Type of Measure “Do the child currently have Point prevalence diarrhea?” “Have the child had diarrhea during Period prevalence the last 3 months?”  Prevalence The number of disease cases in a population at any given time dependent on: ✓ the frequency with which new cases occur and are identified, ✓ average duration of the condition (i.e., time to either recovery or death). Therefore, prevalence may vary from one population to another solely because of variations in duration of the condition. Not very useful when trying to establish and quantify disease determinants Factors affecting Prevalence Incidence of Disease Immigration of cases into the population Death rate related to disease Cure rate Out-migration  Incidence Occurrence of new cases Connected to time ✓ Time of diagnosis Calculations ✓ Absolute incidence ✓ Incidence proportion ✓ Incidence rate  Incidence proportion a.k.a Cumulative incidence IP=A /N over time a specified time period ✓ A: number of cases ✓ N: Population at risk at the beginning of the study period Often used to estimate risk, i.e. the probability that a person will develop disease over time(probability of occurrence of disease in a population during a specified time period)  Incidence rate IR=A /N ✓ A: number of cases ✓ N: Population at risk* time ✓ e.g. 750 cases/100000 person-years Account for changes in the size of the population at risk during the follow-up period New cases related to the person time at risk  Issues in calculating measure of frequency Population at risk must be clearly defined exceptions: eg ca cervix and hysterectomy, measles..vaccination or disease. Choice between risk and rate..prevalence and changes in population at risk may not be significant if disease is rare..  Relationship between incidence and prevalence Prevalence = Incidence × Duration of Disease Illustration of Incidence and Prevalence Thank You.